The present invention relates to a process for regenerating a hydrogenation catalyst, and to hydrogenation processes carried out with a catalyst comprising at least the regenerated catalyst.
The invention relates more particularly to a process for regenerating catalysts of Raney type used in processes for the total or partial hydrogenation of compounds comprising nitrile functions into amine functions.
The hydrogenation of compounds comprising nitrile functions into amine compounds has been carried out industrially for a long time.
Thus, hexamethylenediamine, a compound used in particular for the manufacture of polyhexamethyleneadipamide, also referred to as PA66 and known as xe2x80x9cNylonxe2x80x9d, is manufactured industrially by hydrogenation of adiponitrile in the presence of a Raney nickel catalyst. This process is described in particular in U.S. Pat. No. 3,821,305. Other processes for hydrogenating nitrile or polynitrile compounds into amine compounds are described in U.S. Pat. Nos. 3,372,195, 2,287,219, 2,449,036, 3,565,957, 3,998,881, 4,188,146, 4,235,521, 4,254,059 and WO 95/17959.
These documents relate to the hydrogenation of various aliphatic, aromatic, substituted, unsaturated, etc. nitrile compounds.
These patents also relate to processes which are carried out in the presence of solvent, sodium hydroxide and ammonia. They are generally carried out with Raney-type catalysts such as Raney nickel or Raney cobalt.
The preparation of these Raney catalysts has been described for a long time, and in particular in U.S. Pat. No. 1,638,190 and in J.A.C.S. 54, 4116 (1932). A preparation process starting with nickel, molybdenum and aluminium alloy is described in U.S. Pat. No. 2,948,887.
Raney-type hydrogenation catalysts whose catalytic effect is improved by doping with other metal elements have also been proposed. For example, U.S. Pat. No. 4,153,578 describes a Raney nickel catalyst comprising molybdenum. This catalyst is used in particular for reducing aldehydes to alcohols.
Processes for hydrogenating polynitrile compounds by reduction of certain nitrile functions to give compounds comprising nitrile and amine functions have also been proposed. One application developed is the partial hydrogenation, known as the semi-hydrogenation, of aliphatic dinitriles such as adiponitrile into aminonitriles such as aminocapronitrile. Thus, U.S. Pat. No. 4,389,348 describes the semi-hydrogenation of dinitrile into xcfx89-aminonitrile with hydrogen, in an aprotic solvent and ammonia medium and in the presence of rhodium deposited on a basic support. U.S. Pat. No. 5,151,543 describes the semi-hydrogenation of dinitriles into aminonitriles in a solvent in a molar excess of at least 2/1 relative to the dinitrile, comprising liquid ammonia or an alkanol containing a mineral base in the presence of a catalyst such as Raney nickel and Raney cobalt.
Similarly, patent application WO 93/16034 describes a process for the semi-hydrogenation of adiponitrile into aminocapronitrile in the presence of a Raney nickel catalyst, a base and a transition metal complex.
In the documents cited, the problem of reducing the consumption of catalyst, either by better recovery or by recycling, is not envisaged.
The recovery of the hydrogenation catalyst is envisaged in the case of complete hydrogenation by U.S. Pat. No. 4,429,159, which describes a process for pretreating Raney nickel catalyst with a carbonate to reduce the entrainment of this catalyst in the hexamethylenediamine flow. The catalyst thus recovered can be recycled after washing with water, as a mixture with fresh catalyst.
Processes for regenerating the catalysts used in semi-hydrogenation processes have also been described in patent applications WO 97/37964 and WO 97/37963. The catalysts are treated with a flow of nitrogen at a temperature of between 150xc2x0 C. and 400xc2x0 C., in the absence of any liquid or solvent. After regeneration by treatment with hydrogen, the catalysts are washed with water to neutral pH and optionally conditioned with liquid ammonia. These regeneration processes use a high-temperature treatment which can give rise to partial sintering of the catalyst. In addition, these processes do not make it possible to recover total catalytic activity, in particular when the process for hydrogenating the nitrile compounds is carried out in liquid medium, and more particularly in the presence of a basic compound.
One of the aims of the present invention is to propose a process for regenerating a hydrogenation catalyst of the Raney catalyst type which makes it possible to recover an activity substantially equivalent to that of a fresh catalyst.
To this end, the invention proposes a process for regenerating a catalyst of the Raney catalyst type for the total or partial hydrogenation of nitrile functions into amine functions on organic compounds, characterized in that it consists in mixing the spent catalyst, separated from the hydrogenation reaction medium, with an aqueous solution of a basic compound having an anion concentration of greater than 0.01 mol/l, in maintaining the mixture at a temperature below 130xc2x0 C. and then in washing the treated catalyst with water or a basic aqueous solution until the final pH of the washing waters is between 12 and 13.
According to another preferred characteristic of the invention, the process for regenerating the catalyst can comprise a hydrogenation of this catalyst, carried out by treating the catalyst under a hydrogen atmosphere and at a temperature below 130xc2x0 C.
According to the invention, the catalyst can be placed under a hydrogen atmosphere before maintaining the spent catalyst/basic solution mixture at the reaction temperature. In this case, the basic treatment and the hydrogenation of the spent catalyst are carried out simultaneously.
In another embodiment, the catalyst is hydrogenated before the step of mixing with the basic aqueous solution. Finally, it is also possible to subject the catalyst, treated with a basic solution and optionally washed, to the hydrogenation step.
From the point of view of cost-effectiveness and ease of implementation, the process consisting in simultaneously carrying out the treatment with a basic a solution and hydrogenation is preferred.
The various characteristics and definitions of the products or operating conditions described below are applicable to all the embodiments mentioned above.
The regeneration process of the invention can be carried out in batchwise or continuous mode.
The process of the invention allows a regeneration of the hydrogenation or semi-hydrogenation catalyst at low temperature, thus avoiding deterioration of the catalyst, more particularly a reduction in the doping effect of the metal elements contained in the Raney catalyst.
According to the invention, the catalysts which can be regenerated by the process described above are Raney-type catalysts such as, for example, Raney nickel and Raney cobalt. These catalysts can advantageously comprise one or more other elements, often referred to as dopants, such as, for example, chromium, titanium, molybdenum, tungsten, manganese, vanadium, zirconium, iron, zinc and more generally the elements from groups IIB, IVB, IIIB, VB, VIB, VIIB and VIII of the Periodic Table of the Elements. Among these dopant elements, chromium, iron and/or titanium or a mixture of these elements are considered as being the most advantageous and are usually present at a concentration by weight (expressed relative to the Raney nickel or Raney cobalt metal) of less than 10%, preferably less than 5%.
Raney catalysts often comprise traces of metals present in the alloy used to prepare the said catalysts. Thus, aluminium is especially present in these catalysts.
According to one preferred characteristic of the invention, the basic aqueous solution used to treat the catalyst is a solution of alkaline base or aqueous ammonia. The basic compound preferably has a high solubility in water.
Mention may be made of sodium hydroxide, potassium hydroxide, lithium hydroxide or caesium hydroxide as basic compounds that are suitable.
The concentration of anions in this basic solution is advantageously between 0.01 N and 10 N, preferably between 0.02 N and 7 N.
The concentration by weight of the catalyst in the basic solution is between 5% and 30% relative to the reaction medium.
In variants comprising a treatment with hydrogen, the reaction medium either consists of the basic solution and the catalyst as described above, or consists of the spent catalyst alone, or of the catalyst treated with the basic solution and optionally washed. The reactor is placed under a partial pressure of hydrogen advantageously of greater than 105 Pa.
The reaction medium is then heated to a temperature below 130xc2x0 C., preferably between 100xc2x0 C. and 130xc2x0 C., for a time which is determined experimentally and by precalibration of the process. This time is of the order of a few hours, advantageously between 1 hour and 10 hours.
In order to allow an exchange with the hydrogen contained in the reactor, the reaction medium is advantageously stirred. Needless to say, other processes for supplying hydrogen, such as bubbling or a packing column, may be used to carry out this treatment without departing from the invention. However, one of the advantages of the process of the invention is that it makes it possible to obtain an effective treatment with a high regeneration yield without the need for complex processes to promote the contact between the hydrogen and the catalyst.
After the catalyst has been treated with a basic solution and has optionally undergone a hydrogenation, it is either extracted from the medium or concentrated in this medium by the usual concentration or separation techniques such as, for example, filtration, decantation, centrifugation, evaporation or the like. The catalyst thus separated out or concentrated is subjected to a washing step in order partially to remove the basic solution and also all the impurities and compounds which limit the activity of the catalyst.
This washing operation can be carried out with water or with a dilute basic solution. The reason for this is that the washing operation makes it possible to remove all the impurities, but must maintain the pH of the catalyst at a value above 12, advantageously between 12 and 13. This pH is monitored by measuring the pH of the washing waters.
Thus, in one preferred embodiment, in particular when the regeneration process is carried out continuously, the washing solution is a basic solution with a concentration of between 0.01 N and 0.1 N.
The catalyst thus regenerated can be used either alone or as a mixture with fresh catalyst in processes for the hydrogenation of compounds comprising nitrile functions.
A subject of the invention is also a process for hydrogenating compounds comprising at least one nitrile function into amine compounds, for example as described in patent application WO 95/19959.
This process consists briefly in adding the nitrile compound to a reaction medium in a concentration by weight of between 0.001% and 30% relative to the total weight of the reaction medium.
The reaction medium is a liquid medium which contains at least one solvent. This reaction medium advantageously contains water in an amount preferably of less than or equal to 50%, preferentially between 0.1% and 15%, by weight.
In addition to the water, a solvent of the alcohol and/or amide type can be provided, such as methanol, ethanol, propanol, isopropanol, butanol, glycols, polyols, dimethylformamide or dimethylacetamide. When water is present, this solvent represents 2 to 4 parts by weight per part of water present.
In one preferred embodiment, the reaction medium also comprises the amine whose preparation is targeted by the hydrogenation process. Thus, in the case of the hydrogenation of adiponitrile into hexamethylenediamine, the process is carried out in the presence of hexamethylenediamine.
The concentration by weight of amine is between 50% and 99% by weight relative to the total weight of the solvent in the reaction medium.
The reaction medium advantageously comprises a basic compound when the catalyst is of the Raney type.
This base is present in a concentration of greater than 0.1 mol/kg of catalyst, preferably between 0.1 mol/kg and 10 mol/kg.
The hydrogenation reaction is carried out by maintaining a suitable pressure of hydrogen in the chamber, i.e. a pressure advantageously of between 0.10 MPa and 10 MPa, and a reaction medium temperature of less than 150xc2x0 C., preferably less than or equal to 100xc2x0 C.
The above process is given by way of illustration. The invention can also apply to other processes in which the hydrogenation is carried out in the presence of ammonia, for example.
A subject of the invention is also a process for semi-hydrogenating compounds comprising at least two nitrile functions, in which at least one nitrile function is reduced to an amine function.
Such processes are used in particular for the synthesis of aminonitrile compounds which, on cyclizing hydrolysis, give lactams. These lactams are monomers for the manufacture of homopolyamides.
Thus, one of the industrialized applications is the manufacture of caprolactam, which is a monomer for polyamide 6, obtained by semi-hydrogenation of adiponitrile into aminocapronitrile and cyclizing hydrolysis of this compound into caprolactam.
This process is described in particular in patent applications WO 93/16034, WO 93/12073 and U.S. Pat. No. 4,248,799. This process uses Raney nickel as catalyst.
Thus, in this process, adiponitrile is added to a reaction medium comprising an alkaline base and/or aqueous ammonia, hydrogen, a Raney nickel catalyst and a transition metal complex. The reaction medium can contain a solvent such as an alcohol.
The catalyst is either dispersed in the reaction medium or supported on a fixed bed.
In another embodiment, the Raney catalyst (nickel or cobalt) can be treated with an alkaline alkoxide, it being possible for the hydrogenation reaction to be carried out in the absence of water and in the presence of an aprotic solvent such as tetrahydrofuran, dioxane, aliphatic diamines, alcohols or ethers.
The semi-hydrogenation process can also be carried out using a Raney catalyst such as Raney nickel comprising a dopant element as defined previously.
In this process, the hydrogenation medium comprises water in a proportion of at least 0.5% by weight relative to the total amount of liquid compounds in the reaction medium, aminonitrile and/or diamine formed by the hydrogenation, and unconverted nitrile compound. The reaction medium also comprises a strong inorganic base, i.e. an alkaline hydroxide present in a concentration so from 0.1 mol/kg to 3 mol/kg of catalyst.
In these processes, the catalysts can consist of a mixture of fresh catalyst and of catalysts regenerated according to the process of the invention. It is also possible to use only a regenerated catalyst.
The nitrile compounds which can be totally or partially reduced by the processes of the invention are, by way of example, xcex1,xcfx89-dinitrile compounds comprising a linear or branched aliphatic chain of 1 to 12 carbon atoms, such as adiponitrile, methyl glutaronitrile, ethyl succinonitrile, malononitrile, succinonitrile, glutaronitrile or a mixture of these compounds.
In general, the catalysts which can be regenerated by the process of the invention are advantageously used in combination with a basic compound in processes for hydrogenating nitrile compounds.
Other advantages and details of the invention will emerge more clearly in the light of the examples below, which are given purely as a guide and with no limiting nature.